Monarch butterflies are as fascinating as they are beautiful. They require one kind of plant — the milkweed — for egg-laying and food for the larva, which themselves have a distinctive striped coloration. Monarchs also undergo an amazing seasonal migration to one specific location in Mexico, a behavior that has been the focus of entire scientific careers.

Unfortunately, however, loss of habitat across North America, brought about largely by farming, has brought with it the progressive loss of the milkweed plant, and the loss of as much as 90 percent of the monarch butterfly population present two decades ago. These declining numbers have prompted the U.S. Fish and Wildlife Service to call for adding the insect to the list of endangered species.

According to the Xerces Society for Invertebrate Conservation, the tremendous decline in monarch numbers relates to increased cultivation of genetically modified crops that can withstand herbicides that kill native plants, including the milkweed that is crucial for monarch reproduction. The Society filed a petition on Monday requesting federal protection for the monarch. That petition is currently under review for the next year with the Fish and Wildlife Service.

North American monarchs that reside east of the Continental Divide fly some 3,000 miles to Mexico each autumn. Those west of the Divide migrate a shorter distance to a location in California. In 1996, an estimated one billion monarchs migrated to Mexico. Last year, the number was substantially lower at 35 million, according to ecologist Marcus Kronforst at the University of Chicago.

The monarch migration remains a bit of a mystery. Each year, new adults make the journey with no prior experience. Researchers have discovered that they rely partially on the sun’s position and the Earth’s magnetic field when the sun is obscured by clouds. Exactly how the butterflies find their way to the very same spot year after year remains unknown.